1. Field of the Invention
The present invention relates to a mixed catalytic solid for the (co)polymerization of ethylene, containing both a metallocene and a catalyst of a metal from group VIB of the Periodic Table, in particular chromium. The invention also relates to a process for the preparation of this catalytic solid, to a catalytic system for the (co)polymerization of ethylene comprising this catalytic solid and to a process for the (co)polymerization of ethylene using the catalytic system.
2. Description of the Related Art
Various types of catalysts are known for the polymerization of ethylene, such as, for example, chromium-based catalysts, titanium-based catalysts or metallocenes, the characteristics of the polyethylene obtained being determined in particular by the nature of the catalyst used.
In order to obtain a copolymer of alpha-olefins of wide molecular mass distribution, it has already been proposed in patent application PCT-A-92/15619 to use a two-step process in which at least two alpha-olefins are polymerized in the first step using a metallocene and an aluminoxane, and the polymerization is continued in the second step in the presence of the polymer obtained from the first step. This known process requires a complex plant containing two polymerization reactors connected together.
Moreover, in patent application EP-A-339,571 it is proposed to use a catalyst comprising:
(a) a chromium compound deposited on a porous inorganic oxide support, PA1 (b) a transition metal compound (such as bis(cyclopentadienyl)dichlorozirconium, for example), and PA1 (c) an aluminoxane, PA1 C.sub.p and C.sub.p ' each denote an unsaturated hydrocarbon radical coordinated to the central atom M, it being possible for the groups C.sub.p and C.sub.p ' to be linked via a covalent bridge, PA1 m denotes the transition metal, which is chosen from groups IIIB, IVB, VB and VIB of the Periodic Table, PA1 a, b, x and z denote integers such that (a+b+x+z)=m, x&gt;0, z.gtoreq.0 and a and/or b.noteq.0, PA1 M denotes the valency of the transition metal M, PA1 X denotes a halogen, and PA1 Z denotes a hydrocarbon radical which may optionally comprise oxygen or a silyl radical of formula (--R.sub.t --Si--R'R"R'") where PA1 R denotes an optionally substituted alkyl, alkenyl, aryl, alkoxy or cycloalkyl group comprising up to 20 carbon atoms, PA1 R', R" and R'" are identical or different and each denote a halogen or an optionally substituted alkyl, alkenyl, aryl, alkoxy or cycloalkyl group comprising up to 20 carbon atoms, PA1 t denotes 0 or 1. PA1 MI.sub.2 =melt index of the molten polymer at 190.degree. C., measured under a 2.16 kg load and expressed in g/10 min according to ASTM standard D 1238 (1990). PA1 MI.sub.5 =melt index of the molten polymer at 190.degree. C., measured under a 5 kg load and expressed in g/10 min according to the ASTM standard D 1238 (1990). PA1 HLMI=melt index of the molten polymer at 190.degree. C., measured under a 21.6 kg load and expressed in g/10 min according to the ASTM standard D 1238 (1990). PA1 ST standard density of the polymer, expressed in kg/m.sup.3 and measured according to ASTM standard D 1928. PA1 S=fraction of oligomers in the polymer, expressed in grams of oligomers per kilo of polymer and measured by extraction for 2 hours with hexane at its boiling point. PA1 .mu..sub.2 =viscosity (in dPa.sub.* s) measured at a rate gradient of 100 s.sup.-1. The die used is 30 mm in height and has an internal cross-section of 2 mm. PA1 .mu..sub.0 /.mu..sub.2 =ratio between the viscosities measured at a rate gradient of 1 s.sup.-1 and 100 s.sup.-1. The die used is 30 mm in height and has an internal cross-section of 2 mm. PA1 M.sub.n =number-average molecular mass measured by steric exclusion chromatography performed in 1,2,4-trichlorobenzene at 135.degree. C. on a type 150 C chromatograph from the company Waters. This mass is expressed in daltons. PA1 M.sub.w =weight-average molecular mass measured by steric exclusion chromatography performed in 1,2,4-trichlorobenzene at 135.degree. C. on a type 150 C chromatograph from the company Waters. This mass is expressed in daltons. PA1 M.sub.z =mean molecular mass of the polymer, defined by the relationship: M.sub.z =(.SIGMA. w.sub.i M.sub.i.sup.2)/(.SIGMA. w.sub.i M.sub.i) where w.sub.i denotes the weight of the polymer of molecular mass M.sub.i. The magnitude M.sub.z is measured by steric exclusion chromatography performed in 1,2,4-trichlorobenzene at 135.degree. C. on a type 150C chromatograph from the company Waters. This mass is expressed in daltons.
for the manufacture of polyethylene of wide molecular mass distribution. The use of this known catalyst for the polymerization of ethylene has the drawback of leading, under certain conditions, to polyethylenes which have too low a melt index to be able to be used in processes such as injection, blow moulding, etc.
The present invention is directed towards overcoming the abovementioned drawbacks by providing a novel catalytic solid which makes it possible to obtain polyethylenes of wide molecular mass distribution and of relatively high melt index, which are suitable for simple subsequent use in processes such as injection, etc.